1. Field of the Invention
The present invention relates to filter apparatus for use in clean rooms, mini-environments, and similar facilities, and particularly ultra-clean filters for use in such facilities.
2. Description of Related Art
Filtration efficiencies and contamination control are growing areas of concern in many industries requiring clean and ultra-clean facilities, such as in pharmaceutical and semiconductor manufacture. As tolerances for contamination have become stricter, a variety of improved air conditioning and air filtering products have been developed.
One of the areas of most active interest has been in the development of improved air filters capable of removing sub-micron particulates and other contaminates from the air. High Efficiency Particulate Air (HEPA) filters generally have a minimum efficiency of about 99.97% at 0.3 .mu.m. In more sensitive environments, Ultra-Low Penetration Air (ULPA) filters can be used having a filtration efficiency in excess of 99.999% at 0.1 .mu.m.
One form of HEPA filter is disclosed in U.S. Pat. Nos. 5,158,586 and 5,238,477 both issued to Layton. This filter comprises a membrane formed from either a continuous sheet of porous metal (e.g., stainless steel, titanium, aluminum) having interstices defining minute pores admitting gas but rejecting particles borne by the gas, or a tightly woven sheet of fine wires. Although filter efficiency is not discussed in these patents, it is taught that a steel sheet generally has a thickness of about 0.01 inches and pores of about 0.5 .mu.m.
A number of HEPA and ULPA filters are available commercially, such as from Flanders Filters, Inc., Marietta, NC, American Air Filter Co., Louisville, Ky., Filtra Corp., Hawthorne, N.J., and Donaldson Co., Inc., Minneapolis, Minn. Typically these filters are composed of a fibrous filter media (e.g., fiberglass fibers bound by an acrylic resin) oriented into a pleated construction within a frame. While these filters work well for many applications, they suffer from a number of deficiencies.
First, a concern has been raised that fibrous filter media may be prone to shedding minute fibers under certain conditions. This may not be a concern in most applications, but can become a major contamination issue where extremely low particle counts are required.
Second, many existing filter units are susceptible to chemical attack under certain conditions. For example, hydrofluoric acid used in semiconductor wet process applications can be particularly aggressive to common air filtration media. In those instances where air is recirculated in a clean environment employing such chemicals, many filter products will prematurely breakdown and begin shedding contaminating particles.
Third, many existing filter media are prone to attack by mere moisture in the air system. For example, fiberglass material may experience clogging and breakdown with high levels of moisture or during prolonged liquid exposure.
Fourth, another filter contamination problem is that some filter products "out-gas" during use. This outgassing commonly occurs from various potting materials used to hold the filter media in a frame (e.g., polyurethane) as well as from the filtration media and/or chemical treatments of the filtration media (e.g., fire retardant treatment). Again, in particularly sensitive environments, even small amounts of chemical release from the filtration apparatus itself can be detrimental. Examples of such compounds include boron and phosphorous.
Fifth, it has been shown that operating air systems at greater than 100 ft/min can cause vibration in the filter media. With fibrous filter media, such as fiberglass, this vibration can result in shedding of particles and contamination. Additionally, due to volumetric flow requirements, in order to achieve 90-100 ft/min flow downstream from the filter, a flow rate of 110-120 ft/min or more may be required at the filter face.
U.S. Pat. No. 5,114,447 issued to Davis discloses an ultra-high efficiency porous metal filter with filtration efficiency up to 99.9999999% for use in an in-line supply system for process gas. While this patent demonstrates that extremely high filtration efficiencies are possible, the construction employed to achieve this level of cleanliness is completely inappropriate for use in filtering a large volume of air over a wide surface area in the manner normally used in a room or mini-environment filtration system. Additionally, although the pressure drop caused by such a filter may be only of minimal concern in a high pressure gas line, it is believed to be intolerable for air conditioning/filtration applications.
Accordingly, it is a primary purpose of the present invention to produce an air filter for use in air supply systems for clean environments that are extremely clean while maintaining acceptable pressure drop across the filter media.
It is a further purpose of the present invention to produce an air filter for use in air supply systems for clean environments that does not contribute its own contamination during use, such as by shedding particles or through out-gassing.
It is another purpose of the present invention to produce an air filter for use in air supply systems for clean environments that is chemically inert, allowing it to be used in the presence of a wide variety of chemicals without incurring filter media breakdown or damage.
It is still another purpose of the present invention to produce an air filter for use in air supply systems for clean environments that is resist to damage from water or moisture.
These and other purposes of the present invention will become evident from review of the following specification.